In recent years, amplifiers using a device such as a FET (Field Effect Transistor) circuit element are employed in wireless base stations for sending and receiving communication signals. Previously, amplifiers using a device such as LDMOS-FET and GaAs-FET have been employed. Recently, however, amplifiers using a highly efficient GaN-HEMT (Gallium Nitride High Electron Mobility Transistor) capable of operating at lower power than the previous amplifiers are becoming mainstream. In the explanation hereinafter, an amplifier using a GaN-HEMT device is denoted as a high power amplifier.
However, the aforementioned high power amplifier causes an Idq drift when the amplifier controls the output level of the GaN-HEMT device from high power to low power. In the Idq drift, an idle current (Idq) becomes equal to or lower than a prescribed value. Such an Idq drift causes the gain of the GaN-HEMT device to fluctuate by a large degree.
Although a small gain fluctuation in a general amplifier is compensated in an existing apparatus, a large gain fluctuation caused by the Idq drift is not compensated in the existing apparatus. Therefore, when a large gain fluctuation occurs by the Idq drift, control for compensating the fluctuation becomes unstable. As a result, it takes time until the compensation of the gain fluctuation is completed (as examples of conventional technology, see Japanese Laid-open Patent Publication No. 08-32357, and Japanese Laid-open Patent Publication No. 10-294628).